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Trig Identity Problems

Started by ArchG Apr 11, 2007 at 5:04 PM 1 replies 750+ views

Original Post

Apr 11, 2007 at 5:04 PM

Hello, I'm working on a problem but keep getting stuck near what I think is the end.. I'm looking for someone to either confirm or deny that I'm doing this correctly the problem is Evaluate cos (x) given that cos(2x) = 2cos(x). (I know... this should be easy) Well I went at it from the left side using the double angle formulas...to get * = squared 1 - 2sin*(x) = 2cos(x) Then...I don't know if I do this right, but I use the pythagram(?) 1 - 2(1 - cos*x) = 2cos(x) 1 - 2 - cos*x = 2cos(x) cox*(x) = 2cos(x) Ok...then here I don't know what to do....I probably did something wrong along the way....but..what? Thanks

Zipster

Apr 11, 2007 at 6:04 PM

For starters, homework problems aren't allowed. Second, if the problem statement is claiming that cos(2x) = 2cos(x), then it's just wrong. Third, I'm not even sure what the actual problem is... prove an identity? They don't even give you a correct one!

grhodes_at_work

Apr 11, 2007 at 8:21 PM

The problem isn't to prove an identity. The key is to find a specific value of x for which the given equality is true, and that will yield a cos(x). The partial solution is on the right track, but some algebraic mistakes and incomplete. (If the post was correct, the solution would be one very trivial step more. But, it isn't quite correct.)

That's all the clues. Zipster is correct. Homework problems are off topic here. Forum FAQ

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